Background/Context: Early childhood education research has consistently found evidence of immediate, positive effects on developmental outcomes. Despite these strong initial benefits, follow-up analyses often reveal that these effects fadeout in the years following participation (e.g., Bailey et al., 2017, Clements et al., 2013; Lipsey et al. 2018). Many researchers have proposed that fadeout stems from discontinuities in the learning environment from preschool to kindergarten (e.g., Bailey et al., 2020). Within this framework, children may continue to reap the benefits of high-quality preschool efforts if the quality of the subsequent learning environment adequately promotes continued skill building. Surprisingly, evidence supporting the sustaining environments hypothesis has been mixed. Thus, it remains unknown which characteristics of the elementary school environment may contribute to the sustained benefits of early childhood interventions (e.g., Burchinal et al., 2022). However, the peer composition of children's subsequent classroom environments could be a key factor. Classrooms with a greater share of high-quality preschool attendees may be more academically similar, and require less catchup, than classrooms with a greater share of non-program attendees. Teachers may respond to the overall ability level of their classroom by altering their instruction to focus on more advanced materials. These instructional differences may not only facilitate continued academic growth for program children, but they may also promote skill-building opportunities for non-program children. Purpose/Objective/Research Question: This project examines the association between classroom composition and mathematics achievement among children who participated in the TRIAD study (see Clements et al., 2011), a scale-up evaluation of a preschool math curriculum. Specifically, we examined whether: (1) the proportion of "treated" children in each classroom predicted achievement through first grade, (2) peer mathematics achievement predicted child mathematics ability through first grade, and (3) classroom composition predicted changes in the quality and quantity of classroom mathematics instruction. Setting: The TRIAD Study was a cluster-RCT implemented in Boston, Massachusetts and Buffalo, New York. The current analyses rely on student-level data that was collected beginning in the 2006-2007 and ending in the 2008-2009. Population/Participants/Subjects: Participants were eligible for inclusion in our analyses if they completed all assessments, and neither skipped a grade nor were retained. Of the original sample (N=1305), 93% of participants met inclusion criteria in kindergarten (n=1214) and 84% of participants met the criteria for first grade (n=1095). Overall, participants may be described as predominantly low income (85%), with approximately 53% identifying as Black and 21% as Hispanic. Intervention/Program/Practice: Although this proposal does not report impact estimates, the original study examined the effectiveness of "Building Blocks," a preschool mathematics program that utilizes evidence-based learning trajectories to enhance children's understanding of mathematics. In addition to the program's curriculum, the intervention included extensive teacher coaching and professional development (Clements and Sarama, 2008). Research Design: We rely on data from a cluster-RCT, which involved 42 elementary schools in Buffalo and Boston that had public preschool programs. Schools were blocked by achievement and randomized within block to either the "preschool-only" intervention group, the "follow-through" intervention group, or the treatment-as-usual control group. Data Collection and Analysis: Participants' mathematics achievement was assessed in the fall (pretest) and spring (posttest) of preschool using the research-based early mathematics assessment (REMA; Clements et al., 2008). Mathematics achievement was measured again in the spring of kindergarten and first grade, along with classroom observation to assess the quantity and quality of mathematics instruction (Clements et al., 2008). Peer exposure variables were calculated for each condition. The proportion of treated peers was generated by counting the number of "treated" children, subtracting one to effectively remove each child from their own peer environment, and dividing by the class size. This process was replicated for the control group to create the proportion of control peers, which we treat as a type of "placebo" check for our models focusing on treated peers. We analyzed the association between the peer environment and end-of-the-year mathematics achievement using a series of regressions, whereby each specification builds on the last. The first model strictly examines the relationship between classroom composition and student math achievement (z-scored); the second model incorporates baseline covariates and fixed effects for the blocking group and site; the third model includes baseline covariates and preschool fixed effects (i.e., child's school at the time of assignment); and the fourth model includes baseline covariates, preschool fixed effects, and student math score from the prior year (i.e., end of preschool or kindergarten). Findings/Results: Table 1 presents descriptive statistics of classroom level predictors for the kindergarten and first grade samples. Statistics are provided for each of the three conditions, including the preschool only condition, the follow-through condition, and the control condition. Table 2 presents results from the within-condition regressions examining the relationship between the proportion of treatment or control peers and student math achievement. Estimates in each panel were derived from a series of four models, which were run separately for the kindergarten and first grade sample. Results indicate that having a greater share of treated peers in kindergarten was not predictive of kindergarten math achievement. However, the proportion of treated peers was significantly predictive of first grade math achievement. When baseline covariates and clustering were accounted for (columns 2-3 and 6-7), a ten-percentage point increase in treated peers equated to a 0.04-0.05 standard deviation increase in first grade math achievement. This effect held for the follow-through condition even after individual math ability was controlled (b= 0.04; p<0.05). Finally, the proportion of control peers was not significantly related to math achievement at the end kindergarten and first grade for children in the control group. Conclusions: Preliminary results indicate that the proportion of treated peers was not predictive of kindergarten math achievement. Interestingly, having a greater share of "Building Blocks" peers was associated with significant, albeit small, improvements in first grade achievement. While these results are promising, further analyses will probe the sensitivity of our findings. Additionally, we will examine the relationship between the proportion of treated peers and classroom math instruction, as well as the association between peer math achievement and individual math achievement in kindergarten and first grade.